In irinotecan-treated patients with colorectal cancer who have a UGT1A1 polymorphism that limits SN-38 glucuronidation, increased SN-38 exposure results in higher rates of neutropenia and late-onset diarrhea29,46, the dose-limiting toxicities associated with irinotecan47. The gene discussed is UGT1A1; the disease is neutropenia.